In wavelength division multiplexed optical communication systems, many different optical wavelength carriers provide independent communication channels in a single optical fiber. Future computation and communication systems place ever-increasing demands upon communication link bandwidth. It is generally known that optical fibers offer much higher bandwidth than conventional coaxial communications; furthermore a single optical channel in a fiber waveguide uses a microscopically small fraction of the available bandwidth of the fiber (typically a few GHz out of several tens of THz). By transmitting several channels at different optical wavelengths into an fiber (i.e., wavelength division multiplexing, or WDM), this bandwidth may be more efficiently utilized.
There have been many attempts to develop a compact, high-resolution waveguide demultiplexor or spectrometer for application in areas such as spectroscopy, optical networks and optical links and more particularly optical communication systems. Such a demultiplexor can be extremely critical in wavelength division multiplexing (WDM) links. In these links or networks, each channel is assigned a distinct and unique wavelength for data transmission. Thus, the optical fiber that connects channels in a WDM network carries many discrete wavelength channels and a particular wavelength is selected before the data is received. The data reception can be achieved by combining a wavelength demultiplexor, photodetectors and electronic selection circuitries. In WDM links, many wavelengths are multiplexed and transmitted through a single optical fiber to increase the capacity of the fiber. The receiver must demultiplex the many wavelengths and select the proper channel for reception. In these applications, the requirements on the wavelength demultiplexor are typically: an optical bandwidth &gt;30 nm, a wavelength resolution of a few angstroms, polarization insensitivity, compactness, low loss, low crosstalk, and a low manufacturing cost.
At present, there are many known methods of selecting particular wavelengths, however, none are ideal for the applications outlined above.
Techniques for multiplexing and demultiplexing between a single optical fiber comprising the multiplexed channel and plural optical fibers comprising the plural demultiplexed channels are described in various U.S. patents. For example, multiplexing/demultiplexing with birefringent elements is disclosed in U.S. Pat. Nos. 4,744,075 and 4,745,991. Multiplexing/demultiplexing using optical bandpass filters (such as a resonant cavity) is disclosed in U.S. Pat. Nos. 4,707,064 and 5,111,519. Multiplexing/demultiplexing with interference filters is disclosed in U.S. Pat. Nos. 4,474,424 and 4,630,255 and 4,735,478. Multiplexing/demultiplexing using a prism is disclosed in U.S. Pat. No. 4,335,933. U.S. Pat. No. 4,740,951 teaches a complex sequence of cascaded gratings to demultiplex plural optical signals. U.S. Pat. Nos. 4,756,587 and 4,989,937 and 4,690,489 disclose optical coupling between adjacent waveguides to achieve a demultiplexing function. A similar technique is disclosed in U.S. Pat. No. 4,900,118. Although some of these techniques are better than others, there is a need for a system using grating elements that is relatively inexpensive to manufacture and that is provides reasonable precision.
Wavelength dependent optical elements such as diffraction gratings, for example, an echellette grating, have been known for many years to produce a high-resolution spectrum where the wavelength is a function of the diffracted angle. Thus a single grating can demultiplex many wavelengths. When an incident beam comprising a plurality of wavelengths of light is incident upon a bulk diffraction grating, the light is diffracted by the grating and is separated into sub-beams that can be focused by a lens and received by a plurality of waveguides or detectors. However, providing a grating system wherein an array of optical waveguides is precisely positioned a predetermined distance from the focusing lens to capture adjacent spaced wavelength channels, is not without some difficulties.
The array of optical waveguides must be spaced precisely having a predetermined spacing in order to capture light of a particular set of wavelengths (channels). This spacing corresponds to the spacing of the sub-beams, which are produced by the diffraction grating and focused by the focusing lens, and is determined by the line density of the diffraction grating and the focal length of the focusing lens. If the spacing between adjacent waveguides is too large or too small, waveguides designed to couple with and receive particular wavelengths may couple with other wavelengths or may not couple with an intended wavelength efficiently.
Typically, lenses used in commercial applications have a focal-length tolerance of approximately .+-.2% or greater. The cost of using focusing lenses that are within a smaller tolerance, for example guaranteed to be within .+-.1%, adds significant cost to the manufactured device that some customers are not willing to pay. However, it is also impractical to manufacture waveguide arrays such that each array has unique waveguide spacing designed to match the beam spacing produced by a particular lens.
Furthermore, to efficiently couple light, the waveguide must be at an optimum distance from the focusing lens, which is determined by the focal length of the lens.
These difficulties can be addressed by incorporating an imaging lens which provides transverse magnification to correct the mismatch between the sub-beam spacing and the waveguide spacing and also provides a convenient point of attachment for the waveguide array and a means to position the array at the optimal distance from the focusing lens. For example, if the focal length of the focusing lens is larger than the design value, then the sub-beams will be spaced farther apart than the waveguides. Therefore, the imaging lens would be made to provide a transverse magnification &lt;1 so that the beam spacing is reduced to match the waveguide spacing.
It is therefore an object of this invention to provide a diffraction grating system for separating wavelengths of light wherein compensation is provided to lessen the effects of variation in focal length of a focusing lens.
It is a further object of the invention, to provide a means and method for providing fine adjusting focus control in a wavelength dependent optical system.
It is a further object of the invention to provide a compact, manufacturable wavelength division multiplexing (WDM) device for telecommunications purposes and for other applications that is relatively easy and inexpensive to manufacture.
In accordance with the invention, there is provided a demultiplexing/multiplexing device having a wavelength dependent element for separating an input beam into sub-beams of light in accordance with their wavelength; a plurality of waveguide means having a spacing therebetween for receiving at least some of said sub-beams; a focusing lens disposed between the wavelength dependent element and the plurality of waveguides for focusing said sub-beams a predetermined distance from the wavelength dependent element; and,
an imaging lens disposed between the plurality of waveguides and the focusing lens to compensate for an offset in the focal length of the focusing lens.
In accordance with the invention there is provided, a demultiplexing/multiplexing device having a wavelength dependent element for separating an input beam into sub-beams of light in accordance with their wavelength;
a plurality of waveguide means having a predetermined spacing therebetween for receiving at least some of said sub-beams;
a focusing lens disposed between the wavelength dependent element and the plurality of waveguides, for focusing said sub-beams a substantially a predetermined distance, from the wavelength dependent element; and,
an imaging lens for correcting for an offset in the focal length of the focusing lens.
In one particular embodiment, the imaging lens is nearly a one-to-one imaging lens